Slug Catcher operation

[Indy]

I have a question regarding control on slug catchers.
I understand hat it separates liquid and gas. I have a schematic which shows a flow control valve on the liquid outlet, this is controlled by a flow controller which takes an input from a flow transmitter( on the same line) but this flow controller also takes an input from a level controller( measuring slugcatcher level). Is this just so the level in the tank is maintained at certain level in that the tank won’t empty or is there another reason?

Also with regards to the gas outlet there is a flow control valve on one line which takes an input from a flow transmitter but this flow controller also takes an input from a pressure controller which controls a valve which will send gas to a flare. Am I correct in assuming this only happens during a high pressure scenario and that during normal operations only the gas flow control valve is modulated?

Thanks

 

[LittleInch]

Liquid level is usually maintained at some sort of min level to avoid getting gas into your liquid system. They will need to measure flow upstream of the valve as it will flash off and become two phase down stream of the valve.

Normally you don't want to flare gas as it's a criminal waste of energy. The gas might be needed to keep the flare alight, but ago this should be written in your operating and control philosophy. .

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Indy]

Thanks for the reply. Am I correct in assuming that you also wouldn’t want liquid in the gas stream and this is also why pressure is measured and input into the flow controller ?

Thanks

 

[georgeverghese]

Both control loops described are cascade loops. The LC is the master controller on the liquid outlet, while the PC is the master on the gas outflow loop. Check if there are high set limits programmed in to the corresponding slave controllers- these limits may be the upper limits permissible for gas and liquid.
Unlike override loops acting through a high or low selector, a master controller always operates on the connected slave controller to maintain master controller setpoint.

 

[Indy]

Getting there now I think. So the LC will adjust the fc set point accordingly to maintain the desired level. Also the PC will do likewise with the other FC to maintain the flow set point.

Thanks